In the spin-off media, K9 Mark II remained Romana's faithful companion for many years. In the early 2000s, John Leeson and Lalla Ward featured in a series of audio plays produced by BBV as K9 and "The Mistress", detailing these characters' adventures in a parallel universe. As neither Romana nor E-Space could be licensed, the aliases of the Mistress and the "pocket universe" were used instead.
K9's innards were redesigned twice more over the course of the series, firstly in collaboration with a company called Slough Radio Control. It allowed one of its employees, Nigel Brackley, to be seconded to the series semi-permanently to supervise the prop. Brackley, who has since gone on to a career in the movie industry, controlled K9 for many of its studio appearances. Eventually, there came a point where the dog's inherent liabilities were outweighing his assets, and the internal mechanisms were completely rebuilt by designer Charlie Lumm. The wheels were enlarged and given independent drives for power and better manoeuvrability, and the radio controls were switched from AM to FM signals to resist interference. However, by the time the improved model made his debut in State of Decay, the first story recorded for Season 18, the decision had already been made to write the character out of the series in the adventure Warriors' Gate.
After this he was tricked by Thorne to give him his regeneration unit for a lost memory chip. This turned him into a bomb to make him self-destruct on encountering Jixey. Starkey helped him disarm himself. (TV: Hound of the Korven) K9 Mark 2's brief life was almost cut tragically short when after the fall of the Korven invasion force, Thorne sent the Trojan to kill K9, who defeated it by burning out his power core and exhausting all his energy. As his regeneration unit was in the Trojan, K9 seemingly died at the mansion after his companions bade him farewell. However his regeneration unit homed in at the last minute and resurrected him, giving the frisky pup a new lease on life. The regeneration unit restored his old dog collar so he could celebrate the joyful occasion to the full as he flew off to more adventures, bristling with life. (TV: The Eclipse of the Korven)

After K9 Mark I regenerated into K9 Mark 2, his design was radically altered. His ear probes were replaced with larger silver aerials. A set of blue lights on his head flashed when he spoke. Most of his body was dark silver and a silver dog bone was located under his neck. He seemed to have the same capabilities as Mark I but was presumably more advanced. (TV: Regeneration)
After K9 Mark I regenerated into K9 Mark 2, his design was radically altered. His ear probes were replaced with larger silver aerials. A set of blue lights on his head flashed when he spoke. Most of his body was dark silver and a silver dog bone was located under his neck. He seemed to have the same capabilities as Mark I but was presumably more advanced. (TV: Regeneration)

The Eighth Doctor Adventures novel Interference: Book Two by Lawrence Miles indicates that the Doctor built a Mark IV model sometime prior to the events of that book, but what happened to this unit is not stated. In the novel The Gallifrey Chronicles by Lance Parkin, K9 Mark II makes another appearance, having been trapped within the TARDIS since the events of The Ancestor Cell. At the end of The Gallifrey Chronicles the Doctor sends K9 on a secret mission to Espero, presumably to seek out his former companion, the living TARDIS known as Compassion.
After Mark I sacrificed himself to destroy the Jixen warriors, Starkey retrieved his regeneration unit, which quickly activated. In a blast of white light, K9 was thus reborn as a much more advanced model, though this K9 Mark Z could not access the memories of his previous self, which had been "scrambled". He stayed at the house of Alistair Gryffen, who ran tests on K9, but did not tell the Department. K9 tried to kill Starkey, as he was sprayed with Jixen genetic matter and so registered as a Jixen. Starkey convinced him that he was not an enemy and K9 gave him his dog whistle to summon him when needed. (TV: Regeneration) K9 and the team tracked down the surviving Jixen to Dauntless Prison and killed it. They also faced a Meron and freed the alien prisoners. (TV: Liberation) K9 travelled with Starkey when he was on the run from the Department and rescued Gryffen (who was kidnapped by a Korven). Gryffen later welcomed him and Starkey into his home. (TV: The Korven)

In 1985, a series of gamebooks featuring the Sixth Doctor were published by Severn House under the title Make your own adventure with Doctor Who in Britain and Find Your Fate — Doctor Who in the United States. These books were actually written by scriptwriters for the television series. Martin wrote Search for the Doctor which takes place in the mid-21st century and features K9 Mark IV being reunited with the Sixth Doctor long after Sarah's death.
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